In a cellular communication system, multiple antennas at a base station (BS) and multiple antennas at one or more user terminals (UTs) served by the BS allow two or more independent data streams to be transmitted from the BS to the UT(s) over the same time-frequency interval. The specific transmission technique that makes this possible is referred to as spatial multiplexing. In general, spatial multiplexing is a multiple-input, multiple-output (MIMO) transmission technique that uses the different “paths” or channels that exist between the multiple antennas at the BS and the multiple antennas at the one or more UTs to spatially multiplex the independent data streams over the same time-frequency interval. When one UT is served by the BS, the system is in single-user mode and when multiple UTs are served by the BS at the same time-frequency interval, the system is in multi-user mode.
The number of independent data streams that can be transmitted over the same time-frequency interval can be shown to be limited by the lesser of the number of antennas at the BS and the total number of antennas at the one or more UTs. Further limitations on the number of independent data streams that can be transmitted over the same time-frequency interval result from fast fading, imperfect channel state information (CSI) (which is used to perform spatial multiplexing), and uncorrelated interference.
In T. L. Marzetta, “Noncooperative Cellular Wireless with Unlimited Numbers of Base Station Antennas,” IEEE Transactions on Wireless Communications, vol. 9, no. 11, pp. 3590-3600, November 2010 [Marzetta], a concept referred to as “massive MIMO” was introduced. In general terms, massive MIMO refers to a communication system that has an excess number of antennas available at the BS (and possibly at the UT) that can be used to reduce the effects of fast fading, imperfect channel state information (CSI), and uncorrelated interference when the BS is performing spatial multiplexing. Marzetta showed that when the number of antennas at the BS approaches infinity, the effects of fast fading, imperfect CSI, and uncorrelated interference vanish, leaving only inter-cellular interference caused by pilot contamination. Pilot contamination results from the reuse of pilot sequences in adjacent cells. Marzetta further showed that when the number of antennas at the BS approaches infinity one of the simplest forms of precoding, referred to as matched filtering (MF), becomes optimal for performing spatial multiplexing.
In general, the exact number of excess transmit antennas needed to qualify as massive MIMO is not strictly defined but, by at least one measure, is achieved when multiuser interference and noise become dominated by pilot contamination.
The embodiments of the present disclosure will be described with reference to the accompanying drawings. The drawing in which an element first appears is typically indicated by the leftmost digit(s) in the corresponding reference number.